Many devastating neurodegenerative diseases result from protein misfolding that leads to plaques or inclusions containing the misfolded protein. Despite a recognized central role of protein misfolding, we generally lack insight into the causative molecular events, in part due to the complex etiology of diseases like Alzheimer's Disease (AD). Huntington's Disease (HD) and at least eight other neurodegenerative disorders have been traced to a remarkable well-defined mutation occurring across different genes: the expansion of a pre-existing CAG codon repeat. In HD, this leads to expansion of a polyglutamine (polyQ) tract within the huntingtin protein, with expansion beyond a threshold of ~35 Gln leading to a devastating neurodegenerative disease, with the age of onset dependent on the degree of expansion. HD alone places more than 200,000 Americans at risk of disease, with currently no effective curative or preventative treatments. A dramatic improvement in our knowledge of the misfolding pathway is essential to enable the design of treatments that can ameliorate misfolding, disease onset and toxicity. To address this need, we will deploy state-of-the-art magic-angle-spinning (MAS) NMR spectroscopy. This approach has previously allowed us to characterize various protein aggregates with site-specific and atomic resolution, most recently including an array of polyglutamine-related aggregates. Our past and future success in this endeavor is enabled by an in-depth NMR expertise, exquisite NMR hardware, and highly effective collaborations, which have allowed for key insights into the misfolding process and disease-causing toxicity in HD. Informed by our existing NMR data and mechanistic studies, we hypothesize that there is a critical role for intramolecular collapse into a likely common ?-hairpin conformation. Crucially, this conformational change facilitates self-assembly of the misfolded polyQ into oligomeric and fibrillar aggregates that likely contain a signature structural motif characteristic of the collapsed initial structure. Thus, by studying the misfolded states, we probe the molecular underpinnings of the misfolding by expanded polyglutamine. Using MAS ssNMR we will both characterize and leverage an unusual spectroscopic signature that we hypothesize to reflect a unique internal polymorphism that is characteristic of misfolded polyQ domains. Applying these methods to different disease-related proteins, we test our hypothesis that a common structural mechanism is at work across the polyglutamine disease family. We will examine diseases where previous work suggests qualitative differences in the misfolded structure (and thus misfolding mechanism), and in HD will examine polymorphic aggregates that reportedly have differing toxicities. This work will provide the much needed systematic and detailed characterization of this family of disorders that will not only benefit their treatment, bt will also impact our understanding of structure and toxicity as applied to amyloid-related diseases with more complex etiologies, ranging from AD to various systemic amyloidoses.